The opportunity is once per stress token. For example:

1. Tallon Roll

2. Perform Boost action with Pattern Analyzer

3. Perform Rage action with Experimental Interface

4. Receive four stress. (Tallon Roll, EI, Rage)

5. Measure if enemy ship is within arc at Range One.

6. If so, remove four stress tokens from Nien Nunb.

The opportunity is once per stress token. For example:

1. Tallon Roll

2. Perform Boost action with Pattern Analyzer

3. Perform Rage action with Experimental Interface

4. Receive four stress. (Tallon Roll, EI, Rage)

5. Measure if enemy ship is within arc at Range One.

6. If so, remove four stress tokens from Nien Nunb.

Strictly, you'd measure the moment you did the Rage action, before the stress from EI or the Tallon Roll arrived, but otherwise accurate.

And will be fairly common I think. RAGE/EI/PA is a good build for Nien.